1. Field of the Invention
The present invention relates to an apparatus for reducing noises in a space such as a vehicular compartment or a cabin of a fuselage, or so on.
2. Description of the Background Art
FIG. 1 shows a circuit block diagram of a previously proposed noise reduction controlling apparatus exemplified by a British Patent Application Publication No. 2 149 614 published on Jun. 12, 1985 (corresponding to a Japanese PCT Application Publication No. Heisei 1-501344).
The previously proposed noise reduction controlling apparatus shown in FIG. 1 is applicable to the space such as cabin or like space.
In details, in a space 101, a plurality of loud speakers 103a, 103b, 103c, and a plurality of microphones 105a, 105b, 105c, and 105d are disposed at respective positions of the space. Control sounds are generated from the loud speakers 103a, 103b, and 103c as to interfere with the noise sounds.
Then, residual noises (residual difference noise signals) are measured by means of the microphones 105a, 105b, 105c, and 105d. A signal processor 107 is connected to each of the loud speakers 103a, 103b, and 103c and the microphones 105a, 105b, 105c, and 105d.
The signal processor 107 receives a basic frequency of a noise source measured by basic frequency measuring means and input signals from the microphones 105a, 105b, 105c, and 105d and outputs drive signals to the respective loud speakers 103a, 103b, and 103c so that sound pressure levels within the enclosed space 101 can be minimized.
If the previously proposed noise reduction controlling apparatus disclosed in the above-identified British Patent Application Publication were merely applied to the noise reduction controlling apparatus which reduces noises of a composite input from the periodic signal caused by the engine vibrations and random signal caused by the road surface, the following disadvantages might be raised.
That is to say, in a case where either of the periodic signal and random signal has a higher amplitude than that of the other signal, it is unavoidable that a resolution of a control system needs to be set with reference to the higher amplitude input signal. Therefore, the resolution for the smaller amplitude input is reduced so that a favorable effect of control cannot be achieved.
In addition, it would be possible to perform control using separate (two sets of) signal processors 107 with the periodic signal caused by the engine vibration and random signal caused by the road surface input being picked up by means of respectively separate detectors at different detection points.
However, the whole control system becomes accordingly complicated, becomes expensive, and large sized. Consequently, the apparatus for reducing the noises described above may become unsuitable for that used for the application to the automotive vehicle.